This invention relates to a facsimile receiver which is particularly suitable for, but not limited to, the reproduction of photographic and similar images, by causing a modulated light beam to scan a light sensitive record medium. The invention is an improvement upon the type of facsimile receiver described in U.S. Pat. No. 3,997,722, the disclosure of which is incorporated herein by reference.
The facsimile receiver described in U.S. Pat. No. 3,997,722 employs a helium-neon laser to generate a light beam. An acousto-optical modulator (i) modulates the intensity of the light beam in accordance with the demodulated incoming video signal, and (ii) deflects the light beam longitudinally of a light sensitive dry silver paper to dither the light beam so as to reduce or eliminate visible scan lines in the image formed on the paper. A paper transport moves the paper longitudinally while a rotatably reciprocating scan mirror deflects the light beam to scan across the paper in transverse lines to print the image thereon.
This prior art facsimile receiver suffers from the following deficiencies, among others:
a. As the laser ages, the intensity of the light beam decreases substantially, adversely affecting the printed image.
b. The scan mirror causes the focal point of the light beam to scan along a circular arc, whereas the paper surface lies in a plane. As a result, in each scan line the light beam is in focus at only the two points where the focus arc intersects the paper surface, so that the sharpness of the printed image is reduced.
c. The dry silver paper is heat sensitive as well as light sensitive, so that the size of the "spot" or "pel" on the paper varies with the light intensity changes due to the variations of the video signal, resulting in a decrease of image sharpness.
d. Variations in the video signal carrier amplitude (in the case of amplitude modulation) or period (in the case of frequency modulation) between half-cycles due to transmission noise and ripple result in undesirable patterning visible in the printed image.
An object of the present invention is to provide an improved facsimile receiver of the aforementioned type.